1. Field of the Invention
This invention relates to a light-emitting device, in particular, to the light-emitting device which is provided with a semiconductor light-emitting element and with a wavelength conversion member containing a phosphor.
2. Description of the Related Art
Various kinds of light source devices or light-emitting devices have been proposed, wherein a semiconductor light-emitting element and a fluorescent substance are combined (see for example, JP-A 2005-205195; JP-A 2006-210887; JP-A 2006-73202). Light-emitting devices of this kind are designed such that the fluorescent substance is enabled to absorb excitation light emitted from the semiconductor light-emitting element and to emit light having a different wavelength from that of the excitation light.
A light-emitting device, which is excellent in luminescence and color rendering, is disclosed in JP-A 2005-205195. This light-emitting device employs an optical fiber as an optical waveguide of the excitation light and is constructed such that the optical fiber is provided at its distal end with a fluorescent substance.
JP-A 2006-210887 describes a light-emitting device which is excellent in light extracting efficiency, this light-emitting device being provided with a semiconductor light-emitting element which is capable of emitting excitation light and with a dispersion having phosphor particles dispersed therein. In this light-emitting device, the light to be emitted from the phosphor particles in the dispersion is extracted from the excitation light incidence side of the dispersion.
Further, JP-A 2006-73202 describes a light-emitting device for illumination, which is provided with a semiconductor laser element outputting a laser beam and with a light-guiding plate having a light extracting surface coated with a phosphor and surface-emitting a light.
These light-emitting devices described in these patent publications however are accompanied with the following problems. Namely, in the cases of the light-emitting devices described in JP-A 2005-205195 and JP-A 2006-210887, the excitation light is concentrated at one point of the phosphor, creating a point light source exhibiting high brightness and increasing excitation density, thereby raising various problems such as the decrease of light-emitting efficiency and the deterioration of materials. On the other hand, in the case of the light-emitting device described in JP-A 2006-73202, although it is possible to make lower the excitation density and to realize a surface light-emitting source, the structure thereof essentially includes a light-guiding plate and a cylindrical lens, thus making it a complicated structure requiring a large number of optical parts.
Furthermore, in order to effectively suppress the leakage of excitation light in these light-emitting devices constructed as described above, the thickness of the phosphor thereof required to be increased. As a result, the resorption and scattering of light are caused to increase inside the phosphor, thereby deteriorating the luminous efficiency.
Additionally, since it is impossible to increase the excitation density, a light-emitting device which is capable of outputting a high energy beam cannot be utilized, so that these light-emitting devices cannot be applied to an apparatus requiring a high luminous flux such as a lighting apparatus, an image display apparatus, etc.
It is an object of the present invention to provide a light-emitting device which is low in energy loss due to resorption, has a large light emitting area, can utilize a exited light of a high energy, and can output a visible light of a high luminous flux.